The present invention relates to integrated circuit air bridge structures and methods of fabricating such structures which facilitate the formation of the integrated circuits and components thereof at the substrate level. The invention is especially suitable for use in providing integrated circuits which are hermetically sealed so as to protect the integrated circuits and any components, such as interconnecting conductors air bridges, inductors or capacitors, against damage or contamination from outside the device.
In order to reduce interconnect capacitances in high performance and high frequency processes, air bridges are often used. A typical air bridge is formed using a second layer of interconnect metal deposited and patterned over a sacrificial material. The sacrificial material is later removed to leave a metal lead surrounded by air rather than a dielectric, such as oxide. The capacitances to the substrate and to other metal lead is thus reduced since air has a lower dielectric constant than do solid insulators such as silicon dioxide or silicon nitride.
However, traditional air bridge manufacturing techniques and structures have several disadvantages. The length of an air bridge is often limited by flexure of metal between two vias. So, relatively long air bridges can only be manufactured by stitching together multiple lengths of short air bridges. Another problem is that circuits fabricated with air bridges cannot be passivated. In a normal process, a passivation layer is deposited on top of an integrated circuit. Typical passivation layers are silicon oxide or silicon nitride. However, for air bridge structures, the passivation layer has to be omitted otherwise the passivation layer will fill the air under the bridge and thereby increase the capacitance of the air bridge or damage the bridge itself.
Accordingly, there has arisen a need for air bridges that can be made of longer lengths of metal than are available in air bridges of the prior art and also for air bridges that can be incorporated into integrated circuits where such circuits have a passivation layer.
It is the principal object of the present invention to provide improved integrated circuit air bridge structures which may be fabricated at the substrate level and which are passivated in the course of fabrication thereby avoiding the need for ceramic packaging or encapsulation, as well as to methods for fabrication of such structures.
It is a still further object of the present invention to provide improved integrated circuit air bridge structures which may be fabricated at the substrate level without materially increasing the volume occupied by the integrated circuit and any components.
It is a still further object of the present invention to provide improved integrated circuit air bridge structures having air bridges or other components made out of conductive elements (e.g., inductors or capacitors), wherein sufficient spacing is provided between the air bridges of the components and the active integrated circuit so as to reduce the effect of parasitic capacitance between the conductive elements and the circuits and adversely affecting the high frequency response of these circuits, as well as to methods of fabricating such structures.
The invention may attain one or more, but not necessarily all, of the foregoing objects.
Briefly described, an integrated circuit structure in accordance with the invention provides an air bridge fabricated on the same die as the integrated circuit to which the air bridge is connected.
The invention provides an on-silicon air bridge that is compatible with single substrate and bonded substrate structures. The invention provides an air bridge structure on a semiconductor substrate or a device substrate. The device or semiconductor substrate may have one or more integrated circuits or semiconductor devices formed therein. The air bridge structure comprises an elongated metal conductor that is encased in a dielectric sheath. At least a portion of the sheath is exposed to ambient atmosphere. In one embodiment, the entire sheath is exposed to atmosphere. However, other embodiments expose a substantial portion of the sheath to ambient atmosphere in order to reduce the dielectric coupling between the sheath and the semiconductor substrate. In a typical construction, the encased conductor crosses a cavity in the substrate. The encased conductor is supported in its transit across the cavity by posts that extend from the lower surface of the cavity. The support posts comprise dielectric material, substrate material, or both.
In its broader aspects, the air bridge structure is made by forming a dielectric layer over semiconductor substrate. Next, an elongated conductor is formed over the dielectric layer and is encased in dielectric material. Then, portions of the substrate or the dielectric layer, or both, are removed to expose the encased elongated conductor to air. The method contemplates using sacrificial materials located between the encased conductor in the substrate. Removing the sacrificial material forms an air bridge cavity. The methods of the invention also include removing portions of the substrate in order to form the air bridge cavity.
Particular embodiments of the invention include a cavity formed in the substrate and/or in the dielectric layer on the substrate. The encased conductors extend across the cavity and enter and exit the dielectric layer overlying the cavity.
The invention may also be used with bonded substrates. In a bonded substrate structure, a device substrate is bonded to a handle substrate, typically with an oxide bonding layer. An air bridge structure is formed in the device substrate in several ways. Those skilled in the art will appreciate that trench isolation is a common step used in the formation of devices and bonded substrates. The air bridge of the invention is compatible with the trench forming steps that are typically used in bonded substrates. In one bonded substrate embodiment, trenches are formed down to the oxide bonding layer. The trenches are coated with a dielectric, filled, and planarized. The dielectric layer covers the planarized trenches and elongated conductors are patterned on the dielectric layer over the air bridge trenches. Another dielectric layer covers the patterned conductors in order to encase them in a dielectric. Then the substrate is further patterned and etched to remove material from between the filled air bridge trenches. The final structure provides air bridge conductors encased in a dielectric that is spaced from the bonding oxide layer.
Bonded substrate structures are used to form inductors. In one embodiment, elongated conductors are encased in a dielectric layer that is disposed over a device substrate region located between isolating trenches. Vias are opened in the dielectric layer and substrate material is removed to form an air bridge cavity beneath the encased conductors. Two air bridge cavities may be formed near one another and separated by a third cavity. Over each air bridge cavity conductors are patterned in a continuous, spiral path of metal in order to form an inductors. The third cavity is filled with ferromagnetic material.
Two further embodiments of the invention use a sacrificial layer for forming a cavity beneath an elongated, encased conductor. In one embodiment, a sacrificial layer of polysilicon is formed over a first dielectric layer that is on the semiconductor substrate. An encased conductor is formed over the sacrificial layer. Vias are opened to the sacrificial polysilicon and the polysilicon is removed to leave an air bridge cavity beneath the encased conductor and between the encased conductor and the silicon substrate. In an alternate embodiment, the dielectric layer on the surface of the substrate is partially removed before the sacrificial polysilicon is deposited. The sacrificial polysilicon is removed along with portions of the underlying substrate. The latter provides an enlarged air bridge cavity beneath the encased conductor.
The foregoing and other objects, features and advantages of the invention as well as presently preferred embodiments thereof and the best known techniques for fabricating integrated circuit structures in accordance with the invention will become more apparent from a reading of the following description in connection with the accompanying drawings.